In order to cancel a noise reaching the ear from one direction, a method of creating and adding a control sound with a phase opposite to that of the reaching noise is generally employed. That is, it is a method of canceling the sound by using a wave nature of a sound. That is, even a noise is a sonic wave and thus has a predetermined phase. Therefore, by creating a sound with a phase opposite to that of the noise and adding the sound and the noise, the two sounds cancel each other and the sound is made smaller.
FIG. 1 is a diagram for illustrating a muting technology for muting a noise in a piping. Bidirectional noises are generated in a piping 101. The noises generated in this piping 101 propagate to a piping 102 to become a one-way noise, which propagates from a left side to a right side in FIG. 1. In order to remove such a noise in the piping 102, a microphone 103 for detecting a noise and a microphone 104 for detecting an error signal are provided in the piping 102.
Moreover, an LMS (Least Mean Square) adaptive filter 105 for inputting output signals from the microphone 103 and the microphone 104 is provided, and an amplifier 106 for amplifying an output of this LMS adaptive filter 105 and a muting speaker 107 for inputting an output of the amplifier 106 are arranged. The LMS adaptive filter 105 will be described later, but owing to development of the recent digital signal processing technology, an active muting device using the LMS adaptive filter has been widely used.
With the above described configuration, the noise propagating in the piping 102 is first detected by the microphone 103, and this signal is supplied to the LMS adaptive filter 105. On the other hand, an error signal that the microphone 104 outputs after detecting a muted sound is also supplied to the LMS adaptive filter 105. The LMS adaptive filter 105 is a filter functioning so that the error signal that the microphone 104 outputs after detecting the noise-canceled sound becomes as close to ‘0’ as possible.
The output signal of the LMS adaptive filter 105 is reversed and amplified by the amplifier 106 and supplied to the muting speaker 107. Since the sonic wave outputted from the muting speaker 107 has become a sonic wave with a phase opposite to that of the sonic wave detected by the microphone 103, the noise propagating in the piping 102 is canceled. Therefore, the noise detected by the microphone 104 becomes as close to ‘0’ as possible. Here, the distance between the microphone 103 and the speaker 107 needs to be arranged to be 30 cm or more apart from each other in order to ensure time for signal processing in the LMS adaptive filter 105. Moreover, the sound that can be muted by this method is only a noise propagating in one direction.
An example illustrated in FIG. 2 is an example in which the noise generated from a noise source 200 propagates from three directions including reflections from a floor and a ceiling. Here, the case in which, a reflection noise from a ceiling and a reflection noise from a floor are added in addition to the directly propagated noise, and the noises from three directions propagate in a space will be described. In the example in FIG. 2, too, a microphone 201 for detecting a noise and a microphone 202 for detecting an error signal are provided, and these detected signals are supplied to an LMS adaptive filter 203. Then, an output of the LMS adaptive filter 203 is inputted into an amplifier 204, reversed and amplified and then supplied to a speaker 205. Processing in the LMS adaptive filter 203 is the same as that in the LMS adaptive filter 105 in FIG. 1, and thus the explanation will be omitted.
The noise directly reaches the microphones 201 and 202 along a path of an arrow a from the noise source 200, and the noise is reflected by the ceiling and the floor to indirectly reach the microphone 202 along paths of arrows b and c. As is known form this figure, the distance from the noise source 200 to the microphone 202 is different between the path a and the paths b and c, and thus, a phase of the noise reached along the path a and phases of the noise reached along the paths b and c are different from each other. Therefore, a sonic wave outputted by the speaker 205 by functioning of the LMS adaptive filter into which a signal of the microphone 201 is inputted has a phase opposite to that of the noise detected by the microphone 201. Thus, the noise reached along the arrow a path is canceled, but the noises reached along the arrows b and c cannot be canceled, because the noises do not have the phase opposite to the phase of the sound of the speaker 205. Moreover, since the LMS adaptive filter does not function for the case where the noises are propagated from multiple directions such as other noise sources 200a and 200b, these noises cannot be canceled.
Such a situation is the same in an environment inside an automobile, and thus, it has been extremely difficult to completely remove the noise coming into the automobile.
On the other hand, a device for canceling a noise by generating a noise with a phase opposite to that of a noise applied to an earphone and by adding this opposite-phase signal to a signal from an earphone speaker by using a microphone for detecting a noise arranged around an ear has been proposed as a canceling headphone (see Patent Document 1, for example). In this method, a microphone for converting an ambient noise to an electric signal is provided in a headphone unit covering an ear of a user, and a phase of a noise detected by this microphone is reversed and added to a sound (signal+noise) coming into the ear of the user. As a result, only a sound from which a noise is removed propagates to the ear of the user, and a headphone which cancels an ambient noise (noise-cancels) is realized.
FIG. 3 illustrates an example of a canceling headphone. A microphone 301 for detecting a noise and a speaker (earphone) 302 are provided in a headphone to be worn on an ear, a noise signal detected by the microphone 301 for detecting a noise is converted by a reverse amplifier 303 to an opposite phase and then, supplied to the speaker 302.
Moreover, a noise removing device in which an environmental sound at a hearing position of an acoustic signal is collected by a microphone or the like, and a muting signal created from this environmental sound signal for muting a noise signal component is combined with an acoustic signal to mute the noise at the hearing position is proposed (see Patent Literature 2, for example). The technology described in this Patent Literature 2 is to create a muting signal from an environmental sound signal collected at a hearing position to modulate a carrier frequency with a composite acoustic signal obtained by combining this muting signal and an audible frequency signal (an audio signal, for example) and to supply the modulated signal to an ultrasonic speaker.